Thermohaline Intrusions

Purpose:

To demonstrate the growth of thermohaline intrusions driven by
double-diffusive vertical fluxes.

What Happens:

A long tank is stratified with a vertical gradient of sugar solution
on the left, and salt solution on the right, with equal densities at
the same level on either side. When the barrier separating the two
tank halves is removed and the small internal wave disturbances die
out, a series of intrusive layers forms and grows.

The layers are
thicker near the bottom, where the salt/sugar contrast is larger, and
the speed of advance of the layers is proportional to their
thickness.

Physics of the Phenomenon:

The layers are driven by the density changes that double-diffusive
fluxes create. The feedback cycle is the following:

2. The vertical density flux of the "sugar" fingers causes the sugar
layers to become less dense, and the salty layers to become more
dense, as each layer continues to advect.

3. The changes in density in (2.) above cause the sugary layers to
rise as they advect to the right, and the salty layers to fall as they
advect to the left. This creates the systematic tilt from lower left
to upper right that can be seen.

4. The slight density anomaly of each layer, combined with the tilt,
creates pressure perturbations that drive the advective motions in
step (1.).

Of course, between each finger region is a diffusive region (sugar
solution below salt soution), and if the density flux in the diffusive
regions were to dominate, the layers would slope the opposite way.
This was rarely observed in the laboratory experiments.